I think you misunderstand that for a long time, Id Software has made a lot of money turning 3d engines into a commodity. For John Carmack, the emphasis has always been on developing new graphics technologies that advance the entire gaming industry. He's first and foremost an engineer. He may have done a significant amount of game design back in the day, but for at least a decade he's primarily been focused on pushing the hardware to its limits. Since Doom 3, Id's games have pretty much been tech-demos for their engines. That's not to say that Doom 3, Quake 4, and Rage weren't good games, its just that none of them are truly memorable experiences like their older games were.
There are only a handful of major players that are making meaningful contributions to the world of 3d engines. If companies like Epic, Id, Valve, and Crytek didn't continue to develop new 3d engines, they wouldn't be a commodity anymore.
Also, the Source, Unreal, Cry, and Id Tech 5 engines are still differentiated from one another enough to warrant all four of them existing.
In short, I don't think 3d engines are a "solved" problem. Of course an indie studio looking to publish its first 3d game should obviously use a readily available engine, but the major studios still have good reasons to roll their own.
I'd imagine that a lot of the need for new engines is driven by new hardware, and it seems that the video hardware has stopped evolving at a rapid pace. nVidia and AMD have been focused more on computation than graphics recently.
There are only a handful of major players that are making meaningful contributions to the world of 3d engines. If companies like Epic, Id, Valve, and Crytek didn't continue to develop new 3d engines, they wouldn't be a commodity anymore.
Also, the Source, Unreal, Cry, and Id Tech 5 engines are still differentiated from one another enough to warrant all four of them existing.
In short, I don't think 3d engines are a "solved" problem. Of course an indie studio looking to publish its first 3d game should obviously use a readily available engine, but the major studios still have good reasons to roll their own.